Maximising the accuracy of image-based surface sediment sampling techniques

Recent years have seen increased interest in automated methods, utilizing photographs collected with a hand-held digital camera, for determining the grain-size distribution of coarse river sediments. Such methods are as precise as traditional field methods, and have considerable time and cost advantages. Nevertheless, several unresolved issues pertaining to their deployment remain to be addressed. Using datasets collected from seven gravel-bed rivers, this paper examines four key issues: (i) the minimum area required to obtain a representative sample; (ii) the effect of lower-end truncation on grain-size percentiles; (iii) the effect of river-bed structure such as imbrication and hiding; and (iv) the potential benefits of using individual particle measurements rather than the number (or mass) of particles per size class to calculate percentiles. It is demonstrated that sampling areas of between 50 and 200- times that of the largest grain are adequate to achieve percentile errors of <10% (in mm). The appropriateness of lower-end truncation depends on the study aims and sediment properties. It has a limited effect on higher percentiles, except where sand is a major constituent. Understanding the influence of bed structure on image-derived size information is complicated by the absence of error-free benchmarks against which accuracy may be evaluated, but it is likely that other errors are more important. The use of individual particle measurements to calculate percentiles in preference to classified data is shown to have a small, but appreciable, effect on precision. These results will assist practitioners in making appropriate operational decisions to maximize data quality using image-based grain-size data capture

The natural wood regime in rivers

International audienc

Bedload infilling and depositional patterns in chute cutoffs channels of a gravel‐bed river: The Ain River, France

International audienc

Characterisation of alluvial plains by remote sensing: Cases studies and current stakes

Recent progress in remote sensing is promising significant advances in fluvial sciences particularly in the study of small and medium rivers. Indeed, sensor resolutions are markedly Improving Malthus Allowing for narrower channels to be imaged and acquisition frequencies for airborne methods aussi Have Improved Malthus Allowing for multi-annual surveys. Indeed, sensor resolutions are improving markedly thus allowing for narrower channels to be imaged and acquisition frequencies for airborne methods have also improved thus allowing for multi-annual surveys. Furthermore, satellite data can now be complemented by catchment scale high resolution airborne imagery Orthorectified Often Collected by national agencies: such as the IGN. Furthermore, satellite data can now be complemented by catchment scale high resolution airborne orthorectified imagery often collected by national agencies such as the IGN. Data acquisition can now be Planned on a multi-annual BASIS and ranks of Acquired from a standard survey platforms: such as aircraft, helicopters, ultralight aircraft or unmanned aerial vehicles (UAVs). Data acquisition can now be planned on a multi-annual basis and acquired from a range of platforms such as standard survey aircraft, helicopters, ultralight aircraft or unmanned aerial vehicles (UAVs). The aim of this paper is to Illustrate progress in this field with a series of examples. The aim of this paper is to illustrate progress in this field with a series of examples. Using Both satellite data (SPOT, Landsat and QuickBird) and UAV data, the paper examines a Will ranks of temporal and spatial scales from river Reaches Exceeding ten kilometers to smaller Reaches Where human impacts Have Been Diagnosed. Using both satellite data (SPOT, Landsat and QuickBird) and UAV data, the paper will examine a range of temporal and spatial scales from river reaches exceeding ten kilometres to smaller reaches where human impacts have been diagnosed

Experimental study of the transient motion of floats reproducing floating wood in rivers

The flow of large wood among hydraulic structures in rivers, especially in urban areas, can cause many problems. Despite many statistical, morphological and hydrodynamical studies on this phenomenon, little information is available on the transient motion of floating wood pieces. In this study, we investigate theoretically and experimentally the transient motion of floating particles under a simple acceleration. From a standard advection model we identify a particle characteristic response distance to the flow, noted λ. This key parameter is then measured for different floating particles reproducing wood in rivers (logs without and with idealized roots). We show here the typical value of this parameter as a function of particle streamwise body length for different particle geometries. The influence of roots can be well captured by an equivalent frontal area, regardless of the root pattern. This response distance could provide useful information on the probability of impact on hydraulic structures depending on the floating wood characteristics